Duchenne muscular dystrophy (DMD) is an X-linked neuromuscular disorder caused by mutation of the dystrophin gene with resultant progressive muscle weakness, leading to death usually by young adulthood. It is the most common childhood neuromuscular disorder affecting about 1 in 3,500 males across all ethnic groups. Although no effective treatment for DMD is available at this time, promising and novel therapeutic treatments have emerged for DMD that will require well-designed clinical trials. Regulatory authorities such as the FDA are often more willing to consider biomarkers as surrogate endpoints or even the primary endpoint in rare disorders such as DMD, particularly in short-duration Phase II dose-ranging studies involving limited numbers of patients. As such, the identification of sensitive and reliable biomarkers becomes a high priority. The goal of this application for an ancillary study to a large clinical trial is to discover and validate sensitive and specific serum biomarkers for DMD. We also propose to develop an integrated molecular/clinical database with associated bio-repository to enable biomarker discovery and validation in DMD on an international scale. The parent study is the 422 subject CINRG Longitudinal Natural History Study in DMD funded by the US Department of Education, NIH and Department of Defense, and Parent Project Muscular Dystrophy until 2015. The specific aims of the project are: Aim 1) Collect sera from 422 DMD subjects ages 4 to adulthood enrolled in the CINRG Longitudinal Natural History Study, with two samples collected per patient at 1 year intervals (844 samples total). For 50 subjects starting corticosteroids, samples will be collected at baseline, 6 months and 1 year post treatment initiation. Aim 2) Carry out a broadly based biomarker discovery on 30 DMD patients, with representative age and disease stage, in comparison to 30 age-matched controls using baseline and 1-year samples from Aim 1. Biomarker discovery will include microRNA profiling, metabolomics profiling, cytokine bead-arrays, and nanoparticle proteomics. Aim 3) Validate selected candidate biomarkers in the entire sample cohort, and identify those biomarkers related to a) disease severity, b) disease progression, and c) responsiveness to corticosteroid treatment effect. Aim 4) Construct an integrated molecular/clinical database and link this to the serum biorepository. We hypothesize that sensitive and specific biomarkers indicative of Duchenne muscular dystrophy disease severity, progression, and corticosteroid drug activity can be identified and validated using measurement of circulating microRNAs, cytokine panels, nanoparticle proteomics, and/or metabolomics. This proposed ancillary study is significant for patients with DMD because it will identify sensitive and disease-specific novel biomarkers. The identification of robust biomarkers will have a substantial impact on Phase II clinical trials using biomarkers as endpoints, and will likely to have significant knowledge transfer to other dystrophies.